Method of forming and shaping plasticized mixtures by low to moderate shear extrusion

ABSTRACT

A method for forming and shaping powder mixtures involves compounding, homogenizing, and plasticizing components to form a mixture. The components are powder materials, binder, solvent for the binder, surfactant, and non-solvent with respect to at least the binder, the solvent, and the powder materials. The non-solvent is lower in viscosity than the binder combined with the solvent. The components are chosen to result in improved wet green strength in the subsequently formed green body. The compounding is done by the steps of dry-mixing the powder material, surfactant and binder to form a uniform blend, adding the solvent to the resulting dry blend, and thereafter adding the non-solvent to the blend. The mixture is shaped by passing it through a low to moderate shear extruder, and then through a die to form a green body. The compounding and shaping steps are carried out at a temperature of no greater than ambient temperature. The method is especially suitable for RAM extrusion. Among the advantages, the method results in higher extrusion velocity, throughputs, and wet green strength, without increasing pressures and torques.

This application claims the benefit of U.S. Provisional Application Ser.No. 60/057,693, filed Aug. 27, 1997, entitled IMPROVED METHOD OF FORMINGAND SHAPING PLASTICIZED MIXTURES BY THERMAL ACTIVATION OF THEPLASTICIZING BINDER; and Ser. No. 60/057,695, filed Aug. 27, 1997,entitled IMPROVED METHOD OF FORMING AND SHAPING PLASTICIZED MIXTURES BYMECHANICAL ACTIVATION OF THE PLASTICIZING BINDER, both by DeviChalasani.

FIELD OF THE INVENTION

This invention relates to a method for forming and shaping stiffplasticized powder mixtures containing binder, binder-solvent, andsurfactant, using low-to-moderate shear extrusion techniques. Increasedplasticization for low-to moderate shear extrusion processes isaccomplished by (1) providing a unique mixture composition that allowsfor less solvent to be used, (2) formulating the mixture and extrudingat temperatures at or below ambient, and (3) increasing the shear rateby increasing the extrusion velocity and/or increasing the amount oftotal work input into the mixture. These features result in an increasein wet green strength of the extrudate without proportional increases inpressure or torque.

BACKGROUND OF THE INVENTION

Powder mixtures having a cellulose ether binder are used in formingarticles of various shapes. For example ceramic powder mixtures areformed into honeycombs which are used as substrates in catalytic andadsorption applications. The mixtures must be well blended andhomogeneous in order for the resulting body to have good integrity insize and shape and uniform physical properties. The mixtures haveorganic additives in addition to the binders. These additives can besurfactants, lubricants, and dispersants and function as processing aidsto enhance wetting thereby producing a uniform batch.

A major and ongoing need in extrusion of bodies from highly filledpowder mixtures, especially multicellular bodies such as honeycombs isto extrude a stiffer body without causing proportional increase inpressures. The need is becoming increasingly critical as thinner walledhigher cell density cellular structures are becoming more in demand forvarious applications. Thin walled products with current technology areextremely difficult to handle without causing shape distortion.

Rapid-setting characteristics are important for honeycomb substrates. Ifthe cell walls of the honeycomb can be solidified quickly after forming,the dimension of the greenware will not be altered in subsequent cuttingand handling steps. This is especially true for a fragile thin-walled orcomplex shaped product, or a product having a large frontal area.

Prior rapid stiffening methods involve time-delayed stiffening usingrapid set waxes as disclosed, for example in U.S. Pat. No. 5,568,652,and/or applying an external field such as an electrical, ultrasonic, orRF field at the die exit. All of these methods involve extrusion of softbatches. Historically, for highly filled ceramic mixtures, soft batcheshave lead to better extrusion quality. Attempts to extrude stifferceramic batches with the current batch components, i.e. cellulose etherbinder, lowering the amount of water and/or additives such as sodiumtallowate or sodium stearate have not been very successful because ofthe higher extrusion pressures resulting from collision of finerparticles, and the abrasiveness of the materials involved.

The growing need for thinner webs (1-2 mil)/high density cellularproducts to be extruded to shape necessitates stiffening at the veryinstant the batch exits the die.

The present invention fills the need for instantaneous forming of stiffbatches which is especially beneficial for extrusion of thin walledhoneycombs, and shape retention of extruded bodies at the very instantthe batch exits the die.

SUMMARY OF THE INVENTION

In accordance with one aspect of the invention, there is provided amethod for forming and shaping powder mixtures involves compounding,homogenizing, and plasticizing components to form a mixture. Thecomponents are powder materials, binder, solvent for the binder,surfactant, and non-solvent with respect to at least the binder, thesolvent, and the powder materials. The non-solvent is lower in viscositythan the binder combined with the solvent. The components are chosen toresult in improved wet green strength in the subsequently formed greenbody. The compounding is done by the steps of dry-mixing the powdermaterial, surfactant and binder to form a uniform blend, adding thesolvent to the resulting dry blend, and thereafter adding thenon-solvent to the blend. The mixture is shaped by passing it through alow to moderate shear extruder, and then through a die to form a greenbody. The compounding and shaping steps are carried out at a temperatureof no greater than ambient or room temperature which is generally takento be about 25° C.

The method is especially suitable for RAM extrusion.

Among the advantages, the method results in higher extrusion velocity,throughputs, and wet green strength, without increasing pressures andtorques.

DETAILED DESCRIPTION OF THE INVENTION

This invention relates to a method for forming and shaping stiffplasticized powder mixtures containing a binder, solvent for the binder,surfactant, and a component in which at least the binder, its solvent,and the powder materials are essentially insoluble. This lattercomponent is referred to as the non-solvent, although in actuality,there can be some solubility of the binder and the solvent in thenon-solvent as long as the viscosity of the non-solvent is not changedsignificantly, and the gel that forms as a result of combining thebinder and solvent does not become weak as a result of some dissolutionin the non-solvent. It is preferred that the binder and its solvent becompletely insoluble in the non-solvent. The combinations of mixturecomponents result in improved wet green strength in the green body thanotherwise occurs. Furthermore, this increase in wet green strengthoccurs without proportional increases in forming pressure or mixingtorque. Also, in extrusion of the above mixtures, the shape of theextrudate or green body is retained at the very instant it exits thedie, with no time delay.

Historically, a mixture or batch of a given composition can be madestiff by removing liquids. But extrusion of such stiff batches resultsin proportional increase in extrusion pressures and torque with enhancedflow defects such as e.g. swollen or deformed webs (in honeycombs). Themethod of the present invention enables forming e.g. extrusion of astiff batch without adversely affecting performance such as pressures,torque, and the flow characteristics.

The method involves forming a stiff batch instantaneously during theplasticization stage of mixing the batch. Stiff batches are formedduring plasticization by increasing the binder to solvent ratio in thebatch. This is done by partial removal of the solvent which contributesplasticity to the batch. The batch is supplemented with a component inwhich at least the binder, the solvent, and the powder materials areessentially insoluble, which is termed the non-solvent. The non-solventdoes not contribute plasticity to the batch. The non-solvent compensatesfor the lost solvent. The non-solvent provides the fluidity necessaryfor shaping, while maintaining stiff batches. This is unlike the solventwhich provides both fluidity and acts as a medium for the binder todissolve in, which results in a soft batch.

The increased stiffness is brought about by increasing the gel strengthof the binder in the solvent and the batch. By gelling here is meantthickening that occurs when the binder and its solvent are combined. Thegel strength of the binder is increased by increasing the weight ratioof the binder to the solvent by partial removal of the solvent from thebatch that would historically be present.

Prior to this invention, if the solvent content were to be reduced, thebinder and the batch would be deprived of solvent necessary for completeplasticization of the binder. This results in a very stiff strong bindergel in the solvent and in the batch. This increase in the effectiveconcentration of the binder in the solvent would lead to a proportionalincrease in pressures, torques, and flow defects when these batches areshaped.

In the present invention, formability of the solvent-deprived stiffbatches is enhanced by the use of the non-solvent. The non-solventcomponent of the batch provides the fluidity necessary for extrusionwhile maintaining the stiffness of the binder gel in the solvent. Whilenot wishing to be bound by theory, it is believed that the formabilityis enhanced by the non-solvent being at two critical interfaces: (1) atthe interface between the batch and the wall of the shaping apparatus,e.g. in extrusion this is the die/extruder wall, front end hardware(screen pack, homogenized flow control device), and (2) at theinterfaces between the individual powder particles.

By highly filled mixtures is meant a high solid to liquid content in themixture. For example, the powder material content in the mixture istypically at least about 45% by volume, and most typically at leastabout 55% by volume.

As mentioned previously, the components of the highly filled mixture orbatch are (1) powders or mixtures of powders, (2) binder to bind theparticles, (3) solvent for the binder which imparts plasticity (binderdissolves in the solvent to provide plasticity), (4) the non-solventwith respect to at least the binder, the solvent, and the powdermaterials, and (5) surfactant which functions as a lubricant/dispersantto disperse the powders in the plasticized mixture. When the binderdissolves in a solvent, the resulting gel is visco-elastic, i.e. the gelis characterized by an elastic component which is a measure ofstiffness, and a viscous component which is a measure of the fluidity ofthe system. Elastic component is typical of a solid-like behavior, andviscous component is typical of a fluid-like behavior. In the presentinvention, partial removal of the solvent results in a significantimprovement in the elastic component of the binder-solvent gel comparedto historic batches.

The Powder Material

Typical powders are inorganics such as ceramic, glass ceramic, glass,molecular sieve, metal, or combinations of these.

The invention is especially suitable for use with ceramic, particularlywith cordierite and/or mullite-forming raw material powders.

By ceramic, glass ceramic and glass ceramic powders is meant thosematerials as well as their pre-fired precursors. By combinations ismeant physical or chemical combinations, e.g., mixtures or composites.Examples of these powder materials are cordierite, mullite, clay, talc,zircon, zirconia, spinel, aluminas and their precursors, silicas andtheir precursors, silicates, aluminates, lithium aluminosilicates,feldspar, titania, fused silica, nitrides, carbides, borides, e.g.,silicon carbide, silicon nitride, soda lime, aluminosilicate,borosilicate, soda barium borosilicate or mixtures of these, as well asothers.

Especially suited are ceramic materials, such as those that yieldcordierite, mullite, or mixtures of these on firing, some examples ofsuch mixtures being about 2% to about 60% mullite, and about 30% toabout 97% cordierite, with allowance for other phases, typically up toabout 10% by weight. Some ceramic batch material compositions forforming cordierite that are especially suited to the practice of thepresent invention are those disclosed in U.S. Pat. No. 3,885,977 whichis herein incorporated by reference as filed.

In accordance with a preferred embodiment, one composition whichultimately forms cordierite upon firing is as follows in percent byweight, although it is to be understood that the invention is notlimited to such: about 33-41, and most preferably about 34-40 ofaluminum oxide, about 46-53 and most preferably about 48-52 of silica,and about 11-17 and most preferably about 12-16 magnesium oxide.

The powders can be synthetically produced materials such as oxides,hydroxides, etc., or they can be naturally occurring minerals such asclays, talcs, or any combination of these. The invention is not limitedto the types of powders or raw materials. These can be chosen dependingon the properties desired in the body.

Some typical kinds of powder materials are given below. The particlesize is given as median particle diameter by Sedigraph analysis, and thesurface area is given as N₂ BET surface area.

Some types of clay are non-delaminated kaolinite raw clay, such asHydrite MP™ clay, or Hydrite PX™ clay, delaminated kaolinite, such asKAOPAQUE-10™ (K10) clay, and calcined clay, such as Glomax LL. All ofthe above named materials are sold by Dry Branch Kaolin, Dry Branch, Ga.

Some typical kinds of talc are those having a surface area of about 5-8m² /g, such as supplied by Barretts Minerals, under the designation MB96-67.

Some typical aluminas are coarse aluminas, for example, Alcan C-700series, such as C-701™, or fine aluminas such as A-16SG from Alcoa.

One typical kind of silica is that having a particle size of about 9-11micrometers, and a surface area of about 4-6 m² /g, such as IMSIL™ soldby Unimin Corporation.

In filter applications, such as in diesel particulate filters, it iscustomary to include a burnout agent in the mixture in an amounteffective to subsequently obtain the porosity required for efficientfiltering. A burnout agent is any particulate substance (not a binder)that burns out of the green body in the firing step. Some types ofburnout agents that can be used, although it is to be understood thatthe invention is not limited to these, are non-waxy organics that aresolid at room temperature, elemental carbon, and combinations of these.Some examples are graphite, cellulose, flour, etc. Elemental particulatecarbon is preferred. Graphite is especially preferred because it has theleast adverse effect on the processing. In an extrusion process, forexample, the rheology of the mixture is good when graphite is used.Typically, the amount of graphite is about 10% to about 30%, and moretypically about 15% to about 30% by weight based on the powder material.

Molecular sieves can also be shaped into bodies in accordance with thisinvention. Molecular sieves are crystalline substances having pores ofsize suitable for adsorbing molecules. The molecular sieve can be in thecrystallized form or in the ammonium form or hydrogen form, orion-exchanged with or impregnated with a cation. The molecular sievescan be provided in ion exchanged form or impregnated with cations eitherbefore forming into a body or after the product body has formed. Theion-exchange and impregnation methods are well known processes. Suchtreatments are within the scope of this invention.

Some types of molecular sieves which are preferred for the practice ofthe present invention are carbon molecular sieves, zeolites,metallophosphates, silicoaluminophosphates, and combinations of these.Carbon molecular sieves have well defined micropores made out of carbonmaterial.

The molecular sieves that are especially suited to the invention are thezeolites. Some suitable zeolites are pentasil, such as ZSM-5, Y, such asultrastable Y, beta, mordenite, X, such as 13X, or mixtures thereof.

Any sinterable metal or metal composition can be used in the practice ofthe present invention. Especially suited are iron group metal, chromium,and aluminum compositions, with the preferred iron group metal beingiron. Especially preferred is Fe, Al, and Cr. For example,Fe5-20Al5-40Cr, and Fe7-10Al10-20Cr powders with other possibleadditions are especially suited. Some typical compositions of metalpowders are disclosed in U.S. Pat. Nos. 4,992,233, 4,758,272, and5,427,601 which are herein incorporated by reference as filed. U.S. Pat.No. 4,992,233 relates to methods of producing porous sintered bodiesmade from metal powder compositions of Fe and Al with optional additionsof Sn, Cu, and Cr. U.S. Pat. No. 5,427,601 relates to porous sinteredbodies having a composition consisting essentially of in percent byweight about 5-40 chromium, about 2-30 aluminum, 0-about 5 of specialmetal, 0-about 4 of rare earth oxide additive and the balance being irongroup metal, and unavoidable impurities such as e.g., Mn or Mo, with thepreferred iron group metal being iron. When rare earth oxide is present,the special metal is at least one of Y, lanthanides, Zr, Hf, Ti, Si,alkaline earth metal, B, Cu, and Sn. When no rare earth oxide ispresent, the special metal is at least one of Y, lanthanides, Zr, Hf,Ti, Si, and B, with optional additions of alkaline earths, Cu, and Sn.

In general, the powder material is fine powder (in contrast to coarsegrained materials) some components of which can either impartplasticity, such as clays, when mixed with water for example, or whichwhen combined with the organic binder can contribute to plasticity.

The weight percents of the binder, solvent, and non-solvent arecalculated as superadditions with respect to the non-organic solids bythe following formula: ##EQU1##

The Binder

The function of the binder is to bind the inorganic powders and impartplasticity to the batch when mixed with a solvent. The preferred bindersused in this invention are aqueous based, that is, capable of hydrogenbonding with polar solvents. Examples of binders are cellulosics,starches, poly(vinyl alcohol), poly(vinyl pyrrolidone), gums such asguar gum, xanthan gum, carageenan, etc., alginates, polyethylene oxides,polyamides, and/or polyacrylates. A combination of binder andcross-linking agent can also be used as a binder component (e.g.polyvinyl alcohol with borax, polyacrylates with poly(vinyl alcohol).Hydrophobically modified aqueous binders can also be used.

Especially useful in the practice of this invention are cellulose etherbinders for aqueous systems.

Some typical cellulose ether binders according to the present inventionare methylcellulose, ethylhydroxy ethylcellulose, hydroxybutylmethylcellulose, hydroxymethylcellulose, hydroxypropyl methylcellulose,hydroxyethyl methylcellulose, hydroxybutylcellulose,hydroxyethylcellulose, hydroxypropylcellulose, sodium carboxymethylcellulose, and mixtures thereof. Methylcellulose and/ormethylcellulose derivatives are especially suited as organic binders inthe practice of the present invention with methylcellulose,hydroxypropyl methylcellulose, or combinations of these being preferred.Preferred sources of cellulose ethers are Methocel A4M, F4M, F240, andK75M celloluse products from Dow Chemical Co. Methocel A4M cellulose isa methylcellulose. Methocel F4M, F240, and K75M cellulose products arehydroxypropyl methylcellulose.

The properties of preferred cellulose ether binders such asmethylcellulose are water retention, water solubility, surface activityor wetting ability, thickening of the mixture, providing wet and drygreen strength to the green bodies, thermal gelation and hydrophobicassociation in an aqueous environment. Cellulose ether binders thatpromote hydrophobic association with the non-solvent and hydrogenbonding interaction with the solvent are desirable. Examples ofsubstituent groups that provide hydrophobic association with thenon-solvent are methoxy, propoxy, and butoxy groups. These substituentswhich provide the hydrophobic association also contribute to the gelstrength of the binder. The substituent groups that maximize thehydrogen bonding interaction with polar solvents e.g. water, arehydroxypropyl and hydroxyethyl groups, and to a smaller extenthydroxybutyl groups. This combination of properties enables binders tobe at the interface between the solvent and non-solvent.

Cellulose ethers that provide especially good hydrophobic-hydrophilicbalance are hydroxypropyl methylcellulose, hydroxyethylcellulose,hydroxyethyl methylcellulose, a combination of hydroxyethyl orhydroxypropyl with methyl, ethyl, propyl, and butyl cellulose.

The distribution (random vs. blocking) of the substituent groups alongthe polymer chain also plays a critical role in determining the gelstrength of the binder. Blocky substitution contributes to higher gelstrength relative to random substitution.

Gel strength increases also with an increase in concentration of thebinder in the solvent. The increase in concentration of the binder inthe solvent lowers the thermal gelation temperature.

The organic binder makes up typically about 2-12% by weight, and moretypically about 2-4% by weight of the powder materials.

The Solvent

The solvent provides a medium for the binder to dissolve in thusproviding plasticity to the batch and wetting of the powders. Thesolvent can be aqueous based, which are normally water or water-misciblesolvents; or organically based. Most useful are aqueous based solventswhich provide hydration of the binder and powder particles.

The Non-Solvent

The non-solvent is not a solvent for at least the binder, the solvent,and the powder materials. The non-solvent is lower in viscosity than thebinder-solvent combination. Partial solubility of cellulose etherbinders in the non-solvent would result in increase of viscosity of thenon-solvent, and loss of lubricating properties needed to shape a stiffbatch. This would result in an increase in shaping pressures andtorques. The function of the non-solvent is to provide the fluiditynecessary for shaping, while maintaining the strength of the binder inthe solvent. The non-solvent can have dissolved surfactants, secondarybinders, lubricants, and additives that enhance the Theologicalperformance. The amount of dissolved substances should be so as to notadversely impact the rheology of the mixture.

In case of an aqueous binder system, the non-solvent is hydrophobicrelative to binder in the solvent e.g. water. One preferredbinder-solvent combination is cellulose ether in water. In thiscombination, the non-solvent hydrophobically associates through themethyoxy substituent of the binder. This combination is especiallyadvantageous for cordierite and/or mullite-forming raw material powders.

With aqueous-based binder solvents, such as water, non solvents can bechosen from both synthetic and natural substances.

Examples of such non-solvents are hydrocarbons, silicones, fluorinecompounds, phosphate esters, esters, liquid CO₂, supercritical fluidse.g. supercritical CO₂, and hot water at a temperature above the thermalgelation temperature for a given cellulose ether, and combinations ofthese. When hot water is used as a non-solvent, it is in combinationwith at least one other non-solvent component.

Examples of useful hydrocarbons are alkanes, alkenes, alkynes,cycloaliphatics, synthetic lubricant base stocks (industrial,automotive, agricultural), polyolefins, and aromatics. Examples of thesetypes of materials are paraffinic oils, e.g. mineral oil, hydrogenatedpolybutenes, alpha olefins, internal olefins, polyphenyl ethers,polybutenes, and polyisobutylene.

Examples of esters are synthetic mono and diesters, and natural fattyacid esters (glycerides). Examples of mono and diesters are adipates,phthalates, polyol esters such as trimethylolpropane, andpentaerythritol. Examples of fatty acid esters are natural plant andanimal glycerides such as soybean oil, sunflower, palm, corn, coconut,cottonseed, castor oil, peanut oil, essential oils (rose, jasmine,orange, lime, etc.) soya fatty acid, tallow, bacon grease, lard, andfish oil.

Non-solvents can also be solids as long as they are processed at orabove the melt point of the solid. For example, fatty acids and fattyalcohols of carbon chain length greater than 22 can be used alone or incombination with other non-solvent components.

Some especially useful non-solvents are hydrocarbons, fatty acids havinggreater than 22 carbon atoms in their chains, fatty alcohols havinggreater than 22 carbon atoms in their chains, natural esters having 14or more carbon atoms in their chains, synthetic esters, and combinationsof these.

More advantageous non-solvents are mineral oil, fatty acid glycerides,monoesters, diesters, and combinations of these.

Most preferred are light mineral oil, corn oil, high molecular weightpolybutenes, polyol esters, a blend of light mineral oil and waxemulsion, a blend of paraffin wax in corn oil, and combinations ofthese.

The solvent for the binder can also be made to function as a partialnon-solvent and a partial solvent for the binder through use ofadditives in the batch. For example, in the case of aqueous basedsolvents such as water and a cellulose ether binder, additives that havegreater affinity for water than for the cellulose ether binder,dehydrate the cellulose ether. The additives can be used to shift thesolvent-non-solvent character of the water. The extent of dehydration isdependent on the additive concentration. The solvent/non-solvent balanceof water can be adjusted with the type and concentration of additivessuch as glycerin, corn syrup, maple syrup, sucrose, sorbitol, andelectrolytes such as the salts of alkali and alkaline earth metals.

The Surfactant

The surfactant plays an important role in determining the interfacialproperties between the inorganic powders, between the inorganics andorganics, and between the components of the organic system. Thesurfactant has the greatest influence in determining the gel strength ofthe binder, adhesion of the binder gel to the inorganics, and adhesionof the non-solvent to the binder. It promotes emulsification between thesolvent and non-solvent. The preferred surfactants co-exist with thebinder at the interface between the solvent and non-solvent. In theformed mixture, the surfactant is at least partially miscible in boththe solvent and the non-solvent. It disperses/wets the inorganicpowders.

Typically, the surfactant is suitable if, by itself without othersubstances, it is insoluble in the solvent at room temperature.

Some surfactants that can be used in the practice of the presentinvention are C₈ to C₂₂ fatty acids and/or their derivatives. Additionalsurfactant components that can be used with these fatty acids are C₈ toC₂₂ fatty esters, C₈ to C₂₂ fatty alcohols, and combinations of these.Preferred surfactants are stearic, lauric, oleic, linoleic, palmitoleicacids, and their derivatives, stearic acid in combination with ammoniumlauryl sulfate, and combinations of all of these. Most preferredsurfactants are lauric acid, stearic acid, oleic acid, and combinationsof these. An especially preferred non-solvent for use with this lattergroup of surfactants is light mineral oil.

Examples of some typical batch compositions are given in Table 1 below.

                  TABLE 1                                                         ______________________________________                                                     1        2       3                                               ______________________________________                                        Talc 96-67     40.78      40.79   40.86                                       Glomax LL      26.48      27      29.71                                       Hydrite MP     15.37      --      --                                          K-10           --         14.82   11.69                                       Alcan C701     15.34      15.4    --                                          A-16           --         --      15.74                                       IMSIL          2.03       2       2                                           METHOCEL (F240)                                                                              2.7        2.7     2.7                                         STEARIC ACID   0.6        0.6     0.6                                         LIGHT MINERAL OIL                                                                            9.2        9.2     9.2                                         WATER (%)      23-24      23-24   23-24                                       ______________________________________                                    

The main interactions contributing to stiffness are the interactions ofthe binder with the (1) solvent, (2) surfactant, (3) surfactant-solvent,(4) inorganics-solvent, and (5) inorganics-surfactant-solvent.

Binder-Solvent

Stiffening from the binder-solvent interaction is observed when theconcentration of the binder in the solvent is increased. The binderconcentration can be increased by partial removal of the solvent or byfurther increasing the amount of binder. In this invention, increasingthe binder concentration is done typically by partial removal of thesolvent. When the solvent is partially removed, the binder is deprivedof the solvent necessary for complete plasticization. This results in avery strong binder gel which is very stiff and not overly plastic.Binding of the inorganic particles with the strong binder gel results ina very stiff batch.

Binder-Surfactant and Binder-Surfactant-Solvent

The type and amount of surfactant is important in determining the amountof solvent required to plasticize the binder. If the surfactant makesthe binder too soluble in the solvent, it results in a weak gel. A weakgel can also be formed when the surfactant hinders the binder(chemically, mechanically, or thermally) from solubilizing in thesolvent. This leads to an unplasticized batch.

Binder-Inorganics-Solvent and Binder-Inorganics-Surfactant-Solvent

The type and concentration of inorganics also has an impact on the totalliquid demand, which in turn affects the amount of solvent available tothe binder. Also, the type of surfactant plays a significant role indispersing the particles and affects the total solvent demand for thebatch.

Some useful mixture compositions are in percent by weight based on thepowder materials, about 2% to 50% non-solvent, about 0.2% to 10%surfactant, about 2% to 10% cellulose ether binder, and about 6% to 50%water. More advantageous mixture compositions are in percent by weightbased on the powder materials, about 5% to 10% non-solvent, about 0.2%to 2% surfactant, about 2.5% to 5%C cellulose ether binder, and about 8%to 25% water. It is to be understood that for powder materials of veryhigh surface area, e.g. >20 m² /g, more water is required. However,according to this invention, the amount of water needed is less than theamount that would be needed otherwise.

Batch-Forming Mechanics

For best results, to ensure batch homogeneity and plasticization, thesequence of addition of the various batch components is important. It ispreferred that batch formation take place in two stages prior to theshaping step.

In the first stage or wetting stage of batch formation, the powderparticles, surfactant, and binder are dry mixed followed by addition ofthe solvent such as in a muller or Littleford mixer. The solvent isadded in an amount that is less than is needed to plasticize the batch.With water as the solvent, the water hydrates the binder and the powderparticles. The non-solvent is then added to the mix to wet out thebinder and powder particles. The non-solvent typically has lower surfacetension than water. As a result, it wets out the particles much morereadily than the solvent. At this stage, the powder particles are coatedand dispersed by the surfactant, solvent, and non-solvent.

It is preferred that plasticization take place in the second stage. Inthis stage the wet mix from the first stage is sheared in any suitablemixer in which the batch will be plasticized, such as for example in atwin-screw extruder/mixer, auger mixer, muller mixer, or double arm,etc. Extent of plasticization is dependent on the concentration of thecomponents (binder, solvent, surfactant, non-solvent and theinorganics), temperature of the components, the amount of work put in tothe batch, the shear rate, and extrusion velocity. Duringplasticization, the binder dissolves in the solvent and a gel is formed.The gel that is formed is stiff because the system is verysolvent-deficient. The surfactant enables the binder-gel to adhere tothe powder particles. The non-solvent partially migrates to the exteriorof the particle agglomerates (inter-particle region) and to theinterface between the batch and the walls of the vessel containing it,e.g. mixer, or extruder, or die wall. This results in a batch that isstiff in its interior and lubricated on its exterior.

Thus the batch is a system of particles separated by thesolvent/surfactant/non-solvent bound/glued with the binder gel which isfurther coated with a film of the non-solvent and surfactant. Withoutwishing to be bound by theory, it is thought that the most importantpart of the system is for the binder/surfactant to co-exist at theinterface between the solvent and the non-solvent lubricating fluid. Thebinder and the surfactant at the interface hydrogen-bond with thesolvent and hydrophobically associate with the non-solvent. If thesurfactant displaces the binder from the interface, it results in a softbatch or an unplasticized batch.

The extrusion is done with devices that provide low to moderate shear.For example hydraulic ram extrusion press, which is the preferreddevice, or two stage de-airing single auger are low shear devices. Asingle screw extruder is a moderate shear device. The extrusion can bevertical or horizontal.

As the stiff batch is passed through the extruder and the die, the filmof lubricating fluid in the inter-particle region and at the interfacebetween the batch and the extruder/die wall provides the lubricationnecessary to maintain lower extrusion pressures for a stiff batch. Ifthe mixing process is not high shear in the second or plasticizationstage (e.g., as in a muller mixer, double arm or Auger or RAM), theplasticization will occur during extrusion through the die because thebatch is subjected to high shear the first time through the die, e.g. informing a honeycomb, through the slots and holes. Batch plasticizationin the die is not desirable because it usually results in honeycombswith extrusion defects such as swollen webs, missing webs etc.

This invention provides a method in low shear plasticization processeswhere plasticity is enhanced thermally by mixing batch components andextruding at temperatures at or below ambient. Preferably the batch iscooled to below room temperature, typically to <15° C. The binderhydration is significantly enhanced below room temperature versus roomtemperature and above used in twin screw extrusion. The enhanced abilityof the binder to solvate at the lower temperatures enables the batch toplasticize and promotes the hydrophobic non-solvent fluid to partiallymigrate to the batch-mixer interface. Secondly, a higher concentrationof binder is used in low shear plasticization than with twin screwextrusion batches without proportional increase in pressure; and that,in combination with the lower temperatures, enables hydration of thebinder for extrusion. The relative amount of binder to solvent wasincreased by decreasing the solvent from the historical amount of about30-32 wt. % to about 20-22 wt. % in the batch. This reduction in solventincreases the binder to solvent weight ratio from the current 12-13 to14-15. Thirdly, the low shear homogenization and plasticization can beenhanced by multiple passes of the batch through the extruder, asthrough a synthetic die or a honeycomb di before the batch is extrudedto form the final green extrudate product.

The total extrusion pressure through the die is composed of the pressureto enter the die and the pressure drop through the die. The higherentrance pressure due to the stiff batch is offset by a much largerpressure drop through the die. As a result, the total extrusion pressurethrough the die is no greater than it would be in historic batches.

The lubrication provided by the non-solvent enables the stiff batch toslip at the wall of the die/extruder. As the stiff batch is extruded, atpoints of high shear through the die, the non-solvent is partiallysqueezed out of the batch to the interface between the batch and thewall of the die/extruder. The driving force for the preferentialmigration of the non-solvent versus solvent to the interface is due to(1) the viscosity of the non-solvent being significantly lower than theviscosity of the binder-solvent gel or mixture, (2) the non-solventbeing incompatible with the solvent, i.e. hydrophobic relative to it inthe case of aqueous based solvents, and (3) the solvent being held bythe binder and inorganics by hydration as opposed to the non-solventwhich is free to migrate.

An unexpected benefit of this invention is that shaping e.g. extrusioncan be done at significantly lower temperatures than was previouslypossible.

The extrudability benefits of this invention are (1) cell orthogonalityat the perimeter of the shaped body, and (2) smooth skin.

The stiff batches of this invention exhibit good shape retention at thedie. In the case of multicellular structures, the cell orthogonality atthe perimeter of the part closer to the skin is greatly improved.

A component of extrudability that is affected by the stiff batches isskin deterioration. Very stiff batches at lower extrusion velocities(<1.52 cm/sec or <0.6'/sec) tend to diverge and split apart as the batchexits the die. Skin deterioration can be overcome by increasing theshear rate during plasticization. Shear rate can be increased in theprocess by increasing the extrusion velocity. In the RAM, extrusionvelocity can be increased by increasing the piston speed.

The higher shear rate/extrusion velocity is the driving force for thenon-solvent to partially migrate to the batch-wall interface providingsmooth skin. Very stiff batches from this invention show superior smoothskin flow at higher extrusion velocities, that is >1.52 cm/sec(>0.6'/sec).

Furthermore, the invention provides a method in which cellulose etherbinders (in water as the solvent) can be processed with higher gelstrength at higher velocities (throughputs). This is in direct contrastto the historical batches where the higher throughput capability wasachieved through the use of cellulose ethers with low gel strength,which translates to drying blisters during dielectric drying.

Another advantage of the present invention is that it provides a meansto instantaneously increase wet green strength without an associatedincrease in mixing torque or extrusion pressure characteristic ofhistorical methods. That is, for a given extrusion pressure or torque,the present invention provides markedly higher wet green strength. Thisis a key to improving the extrusion capability of thin web, high celldensity substrates. The corollary of this attribute is that a reductionin extrusion pressure can be realized for a given green wet strength.This is an important advantage because it makes higher throughputspossible. In general, the maximum throughput is limited by the powerdensity that can be generated by the extruder. In historic methods aswater is removed from the batch the green wet strength increases andthroughput can be increased by increasing the extrudate velocity, buteventually a point is reached where the extruder can no longer generatesufficient pressure and thus the throughput limit is reached. Thepresent invention reduces the extrusion pressure and torque such that asignificantly higher throughput can be realized before the pressurelimit of the extruder is reached. This means that for a given product,even those where an increase in green wet strength is not needed, e.g.,in honeycombs with relatively thick walls, such as about 0.14 mm (7mils) or greater, the present invention enables a higher maximumthroughput when compared to historic methods. This increase offers thepotential to both increase production capacity and reduce productioncosts.

Still another advantage of the invention is that it decreases the wearon the extrusion die, and screw elements, thus extending their life.

The present invention is especially advantageous for PAM extrusion ofhoneycombs at velocities of typically about 2.54-12.7 cm/sec (1-5"/sec).

The bodies of this invention can have any convenient size and shape andthe invention is applicable to all processes in which plastic powdermixtures are shaped. The process is especially suited to production ofcellular monolith bodies such as honeycombs. Cellular bodies find use ina number of applications such as catalytic, adsorption, electricallyheated catalysts, filters such as diesel particulate filters, moltenmetal filters, regenerator cores, etc.

Generally honeycomb densities range from about 235 cells/cm² (1500cells/in²) to about 15 cells/cm² (100 cells/in²). Examples of honeycombsproduced by the process of the present invention, although it is to beunderstood that the invention is not limited to such, are those havingabout 94 cells/cm² (about 600 cells/in²), or about 62 cells/cm² (about400 cells/in²) each having wall thicknesses of about 0.1 mm (4 mils).Typical wall thicknesses are from about 0.07 to about 0.6 mm (about 3 toabout 25 mils), although thicknesses of about 0.02-0.048 mm (1-2 mils)are possible with better equipment. The method is especially suited forextruding thin wall/high cell density honeycombs.

The intrinsic material stiffness or wet green strength of this inventionis typically about 2-2.5 times greater than with historic mixtures.

In addition to the stiffness of the batches, another important advantageof this invention is that there is improved shape retention of the greenbody. Shape retention is especially advantageous in forming complexstructures. Shape of thin-wall cellular substrates e.g. 3-6 mil or less,is maintained at higher extrusion velocities for RAM extrusion.

Stiffening is important for honeycombs having a large frontal area. Forexample, honeycombs of typically about 12.7-22.9 cm (5-9") diameter andlower cell density and very thin walls, e.g. 0.07-0.12 mm (3-5 mils) aremore vulnerable to deformation as they leave the extrusion die. Inaccordance with this invention, there is no cell distortion at theperimeter, and there is significant improvement in shape. Therefore therapid stiffening effects of the present invention are especiallyadvantageous for those types of structure:

The extrudates can then be dried and fired according to known techniquesexcept that drying times will be shorter due to less water in theextrudate. Also, less drying energy is required than for historicbatches. This is especially advantageous in dielectric dryingoperations.

The firing conditions of temperature and time depend on the compositionand size and geometry of the body, and the invention is not limited tospecific firing temperatures and times. For example, in compositionswhich are primarily for forming cordierite, the temperatures aretypically from about 1300° C. to about 1450° C., and the holding timesat these temperatures are from about 1 hour to about 6 hours. Formixtures that are primarily for forming mullite, the temperatures arefrom about 1400° C. to about 1600° C., and the holding times at thesetemperatures are from about 1 hour to about 6 hours. Forcordierite-mullite forming mixtures which yield the previously describedcordierite-mullite compositions, the temperatures are from about 1375°C. to about 1425° C. Firing times depend on factors such as kinds andamounts of materials and nature of equipment but typical total firingtimes are from about 20 hours to about 80 hours. For metal bodies, thetemperatures are about 1000° C. to 1400° C. in a reducing atmospherepreferably hydrogen. Firing times depend on factors as discussed abovebut are typically at least 2 hours and typically about 4 hours. Forzeolite bodies, the temperatures are about 400° C. to 1000° C. in air.Firing times depend on factors as discussed above but are typicallyabout 4 hours.

To more fully illustrate the invention, the following non-limitingexamples are presented. All parts, portions, and this percentages are ona weight basis unless otherwise stated.

The parameters used for comparing the examples of this invention tohistorical batches are extrusion pressures of a rod and ribbon, andribbon stiffness (Load/Deformation (L/D)). The batches are mixed in atorque rheometer/Brabender and extruded through a capillary rheometerinto rods and ribbons.

The intrinsic material stiffness or wet green strength used in thisinvention is referred to as the Load versus Deformation (L/D). Stiffness(L/D) was measured by first extruding ribbons about 3.1 mm (1/8") thickusing a capillary rheometer. The ribbon stiffness was then measured byapplying a load to a piston at a given velocity and measuring thedeformation of the ribbon. The ratio of the load to deformation is ameasure of intrinsic material stiffness. The higher the L/D, the stifferthe batch.

A second measure of stiffness used for measuring the wet strength ofhoneycombs is referred to as the ball drop stiffness. The ball drop testwas carried out using a Fischer Scientific Penetrometer which wasadapted for this test. A ball of a given weight which is fastened to thebottom of the plunger rod is released and allowed to drop onto the testsample. The indentation made by the ball on the sample is measured. Thedegree of indentation is an indication of the stiffness or lack thereofof the sample. Lower ball drop numbers at a given load correspond tostiffer batches, with numbers less than about 80 being considered astiff batch.

The results are summarized in Tables 2, 3, 4 and 5 for each composition.

Table 2 shows the comparison of extrusion pressure and the materialstiffness obtained in this invention vs. the historic batches. Thehistoric or non-inventive batch is referred to as the "Control." Theresults show that the extrusion pressure of rods and ribbons are equalto or lower than the control within experimental error. The materialstiffness (L/D) of this invention is 2X times greater than the control.In other words, this invention allows a stiffer batch to be extruded atequivalent or lower extrusion pressures to the control.

Table 3 shows stiffness and deformation data that is representative ofinventive as well as non-inventive examples for extruded wet honeycombs,e.g. 62 cells/cm² with 0.1 mm thick wall (400 cells/in² and 4 mil wall),measured at the very instant the batch exited the die. Table 3 showsthat the inventive batches are stiffer as shown by lower ball dropnumbers and are extruded with lower extrusion pressure than the controlbatches. Furthermore, the cell collapse at the perimeter of thehoneycomb showed significant improvement over the non-inventive example.

Table 4 shows an example of a non-solvent phase which comprises of lightmineral oil and paraffin wax emulsion. Comparison of the extrusionpressures and the ball drop stiffness shows the current inventive batchto be much stiffer than the control while maintaining lower extrusionpressure.

The fired properties of the material from the present invention arecompared to the control in Table 5. A significant change in the physicalproperties was observed in porosity. An unusual aspect of this inventionis that an increase in porosity is observed over historical batcheswithout a proportional decrease in MOR, and with no significant changesin thermal expansion.

                  TABLE 2                                                         ______________________________________                                        Capillary Rheometer Evaluation                                                Extrusion    Control           This Invention                                 ______________________________________                                        Parameters   Talc 96-67 40.86  Talc 96-97                                                                             40.86                                              Calcined clay                                                                            32.6   Calcined clay                                                                          32.6                                               Hydrous clay                                                                             12.82  Hydrous clay                                                                           12.82                                              A-16 SG    13.72  A-16 SG  13.72                                              (Alumina)         (Alumina)                                                   Methocel F240                                                                            4.0    Methocel F240                                                                          3.0                                                Sodium     1.0    Stearic Acid                                                                           1.0                                                Stearate          Water    23                                                 Water      29.5   Light    12                                                                   mineral                                                                       oil                                            Rod Extrusion                                                                 Pressure (Kg)           249             201                                   Ribbon Extrusion                                                              Pressure (Kg)           144             146                                   Stiffness measurements                                                        (L/D, Kg/mm)                                                                  Load (Kg)               2.07            2.52                                  Deformation (mm)        1.17            0.7                                   Stiffness                                                                     Load/Deformation        1.76            3.6                                   (L/D, Kg/mm)                                                                  L/D - Stdev. (n = 5)    0.06            0.27                                  ______________________________________                                         Data applicable to large ram extrusion (62 cells/cm.sup.2, .096 mm thick      wall, 10.16 cm diameter)                                                      Raw materials mixed in a torque rheometer @ <15° C.               

                  TABLE 3                                                         ______________________________________                                        Ram Stiffening Round Honeycomb                                                Extrusion                  This                                               Comments                                                                              Control            Invention                                          ______________________________________                                        Parameters                                                                            Talc 96-67                                                                              40.86    Talc 96-97                                                                            40.86                                              Calcined  32.6     Calcined                                                                              32.6                                               clay               clay                                                       Hydrous   12.82    Hydrous 12.82                                              clay               clay                                                       A-16 SG   13.72    A-16 SG 13.72                                              (Alumina)          (Alumina)                                                  F240      4.0      F240    3.0                                                Sodium    1..0     Stearic 1.0                                                Stearate           Acid                                                       Water     30.3     Water   22.8                                                                  Light   11.6                                                                  mineral                                                                       oil                                                Pre-              2100-            1700- lower                                extrusion         2350             2000  pressure                             Pressure                                 with in-                                                                      ventive                                                                       sample                               Stiffness                                                                     Die 1                                                                         Ball drop                                                                     (100 g                                                                        LOAD)                                                                         (first piece =    106,96,          62,66,                                                                              more                                 @ die)            114,107,         68,69 stiffness                                              109                    with in-                             (last piece       89               62,57,                                                                              ventive                              of the push)                       66,67 sample                               Die 2                                                                         (first piece =    115,117,         66,66,                                                                              more                                 @ die)            115              56    stiffness                            (last piece       98,97            51,59,                                                                              with in-                             of the push)                       61    ventive                                                                       sample                               ______________________________________                                         All inorganics screened to -200 mesh                                          62 cells/cm.sup.2, .096 mm wall, 10.16 cm diameter                       

Tables 4 and 5 show also that inventive batches are stiffer as shown bythe lower ball drop numbers and are extruded with less pressure than thecontrol batches.

                  TABLE 4                                                         ______________________________________                                                                         This                                                                          in-                                                                   Control vention                                      Ingredients              Percent Percent                                      ______________________________________                                        Inorganics                                                                    Talc 96-67               40.86   40.86                                        Calcined clay            32.6    32.6                                         Hydrous clay             12.82   12.82                                        A-16 SG (Alumina)        13.72   13.72                                        Organics                                                                      Methocel F240            4       4                                            Sodium Tallowate         1       --                                           Stearic acid             --      0.5                                          Light Mineral oil        --      14                                           Paraffin wax emulsion    --      1                                            Water                    31      18                                           Extrusion (Web thickness - 0.02 cm, Mask -2.54 cm))                           Die Pressure (psi)       920     680                                          Ball drop Stiffness (× 10-3 mm)                                                                  43,46,  27,37,                                                                46,45,  27,33                                                                 46                                                   ______________________________________                                    

                  TABLE 5                                                         ______________________________________                                        INTRINSIC MATERIAL PROPERTIES of FIRED SUBSTRATES                             PHYSICAL                   THIS                                               PROPERTIES  CONTROL        INVENTION                                          ______________________________________                                                    TALC 96-67  39     TALC 96-67                                                                             39                                                Calcined clay                                                                             15     Calcined clay                                                                          15                                                Hydrous clay                                                                              20     Hydrous clay                                                                           20                                                Alcan A17   10.5   Alcan A17                                                                              10.5                                              Al-trihydrate                                                                             10.5   Al trihydrate                                                                          10.5                                              Imsil       5.0    Imsil    5.0                                               A4M         4.0    F240     4.0                                               Sodium stearate                                                                           1.0    Stearic acid                                                                           1.75                                              Water       25     Water    18.0                                                                 Light mineral                                                                          7.0                                                                  Oil                                            CTE*10-7/° C.                                                                      5.0                3.9                                            MOR (psi)   3398 ± 184      3194 ± 59                                   % Total Porosity                                                                          30.1               34.7                                           Median Pore size (um)                                                                     2.67               2.0                                            ______________________________________                                    

It should be understood that while the present invention has beendescribed in detail with respect to certain illustrative and specificembodiments thereof, it should not be considered limited to such but maybe used in other ways without departing from the spirit of the inventionand the scope of the appended claims.

What is claimed is:
 1. A method for forming and shaping powder mixtures,the method comprising:a) compounding, homogenizing, and plasticizingcomponents to form a mixture, the components comprising powdermaterials, binder, aqueous based solvent for the binder, surfactant, andnon-solvent with respect to at least the binder, the solvent, and thepowder materials, wherein the non-solvent is lower in viscosity than thebinder combined with the solvent, the components being chosen to resultin improved stiffness in the subsequently formed mixture and green body,the compounding being done by the steps ofi) dry-mixing the powdermaterial, surfactant and binder to form a uniform blend thereof, ii)adding the solvent to the resulting dry blend, and thereafter iii)adding the non-solvent to the blend resulting from step ii; and b)shaping the mixture resulting from step a by passing the mixture througha low to moderate shear extruder, and then through a die to form a greenbody, wherein steps a and b are carried out at a temperature of nogreater than ambient temperature.
 2. A method of claim 1 wherein thecomponents in step a are maintained at a temperature no greater thanabout 15° C.
 3. A method of claim 1 wherein the mixture is shaped bybeing passed through a RAM extruder.
 4. A method of claim 3 wherein themixture is passed through a RAM extruder at a RAM speed that is greaterthan it would be absent the binder, solvent, non-solvent, andsurfactant, and compounding of claim 1, step a.
 5. A method of claim 1wherein the solvent is water.
 6. A method of claim 1 wherein the bindercomprises a cellulose ether.
 7. A method of claim 6 wherein thecellulose ether is selected from the group consisting ofmethylcellulose, methylcellulose derivatives, and combinations thereof.8. A method of claim 6 wherein the binder comprises a cellulose ether,and the solvent is water.
 9. A method of claim 8 wherein the bindercomprises additional components selected from the group consisting ofstarch, poly(vinyl alcohol), poly(acrylic acid), and combinationsthereof.
 10. A method of claim 1 wherein the non-solvent comprisescomponents selected from the group consisting of hydrocarbons, fattyacids with chain length of greater than C₂₂, fatty alcohols with chainlength of greater than C₂₂, natural esters with chain length of C₁₄ orgreater, synthetic esters, and combinations thereof.
 11. A method ofclaim 10 wherein the non-solvent comprises components selected from thegroup consisting of mineral oil, fatty acid glycerides, monoesters,diesters, and combinations thereof.
 12. A method of claim 10 wherein thenon-solvent comprises components selected from the group consisting oflight mineral oil, corn oil, high molecular weight polybutenes, polyolesters, wax emulsion-light mineral oil blend, and combinations thereof.13. A method of claim 8 wherein the non-solvent comprises water, thewater having dissolved additives selected from the group consisting ofsugar, glycerin, and combinations thereof.
 14. A method of claim 1wherein the surfactant comprises components selected from the groupconsisting of fatty acids of C₈ to C₂₂ chain length, derivatives offatty acids of C₈ to C₂₂ chain length, and combinations thereof.
 15. Amethod of claim 14 wherein the surfactant is selected from the groupconsisting of stearic acid, stearic acid and ammonium lauryl sulfate,lauric acid, oleic acid, and combinations thereof.
 16. A method of claim14 wherein the surfactant comprises additional components selected fromthe group consisting of fatty esters of C₈ to C₂₂ chain length, fattyalcohols of C₈ to C₂₂ chain length, and combinations thereof.
 17. Amethod of claim 1 wherein the non-solvent is light mineral oil and thesurfactant is selected from the group consisting of lauric acid, stearicacid, oleic acid, and combinations thereof.
 18. A method of claim 8wherein when the binder is a cellulose ether and the solvent is water,for a given powder material, the pressure of the mixture entering thedie is higher than it would be and the wall shear stress through the dieis lower than it would be absent the binder, solvent, non-solvent, andsurfactant of claim 1, step a, whereby the total extrusion pressure isno greater than it would be absent the binder, solvent, non-solvent, andsurfactant of claim 1, step a.
 19. A method of claim 1 wherein for agiven extrusion pressure, the wet green strength of the green body isgreater than it would be absent the binder, solvent, non-solvent, andsurfactant, and compounding of claim 1, step a.
 20. A method of claim 1wherein for a given wet green strength produced in the green body, theextrusion pressure needed to produce said given wet green strength islower than it would be absent the binder, solvent, non-solvent, andsurfactant, and compounding of claim 1, step a, whereby the throughputis greater than it would be absent the binder, solvent, non-solvent, andsurfactant, and compounding of claim 1, step a.